(1) Field of the Invention
The present invention relates to an image forming apparatus, and particularly to an image density correction technology for correcting density and tint of a reproduced image in the image forming apparatus.
(2) Description of the Related Art
An image forming apparatus employing an electrophotographic system has the following structure. A surface of a photosensitive member is uniformly charged by a charger, and the charged surface of the photosensitive member is scanned by laser light to form an electrostatic latent image. Then, toner is supplied to the electrostatic latent image by a developer to develop the electrostatic latent image. A toner image resulting from the above process is primarily transferred onto an intermediate transfer member such as an intermediate transfer belt, and then is secondarily transferred onto a recording sheet to form an image.
By the way, an amount of toner attached to the photosensitive member and a transfer rate of toner onto the intermediate transfer belt are greatly influenced by an environment of the image forming apparatus such as temperature and humidity, variation in charging characteristics due to deterioration of toner over time, and so on. This sometimes changes the density of a reproduced image, and particularly changes the tint of the reproduced image of color image to the great extent.
In response, control for automatically correcting the image density has been conventionally performed, as part of image stabilization processing.
The following describes the details of such processing. A toner pattern is formed on the intermediate transfer belt in accordance with a predetermined timing. The formed toner pattern is irradiated with light from a light source, and its reflected light, that is specular reflected light or diffuse reflected light, is detected by a photoelectric sensor. Hereinafter, detection on tonner patterns means detection of intensity of light reflected from toner patterns. When a toner density obtained based on a detected value differs from a desired density, the control is performed such that a reproduced image has an appropriate density. The control is performed by controlling a charge potential by the charger on a surface of the photosensitive member, output of laser light, developing bias applied to a developing roller, gradation conversion curve (γ curve), and so on, and is hereinafter referred to as “image density correction processing”.
An intermediate transfer belt included in an image forming apparatus is generally endless, and is molded by injection-molding or centrifugal-molding. Demolding process results in linear scratches (hereinafter referred to as “manufacturing-derived scratches”) on a surface of the intermediate transfer belt.
In response, process of removing such manufacturing-derived scratches on the surface of the intermediate transfer belt has been performed (hereinafter referred to as “shape correction process”). However, since this shape correction process increases the manufacturing costs, and sometimes decreases the manufacturing yield, there is recently a demand for omission of the shape correction process upon request for cost reduction.
If the shape correction process is omitted, show-through of the manufacturing derived scratches appears on a toner pattern formed in the above image density correction processing, and this deteriorates the detection precision of toner density. Particularly when detection is performed on a half-tone toner pattern, there occurs markedly such a negative effect caused by the show-through. This might make it difficult to perform precise image density correction processing.
On the other hand, a time required for the image density correction processing is desirably as short as possible, in order to avoid delay of outputting the first page when the image density correction processing is performed immediately before a print job is executed, or in other case, in order to respond to a demand for reducing downtime.